The toggle circuit failed fairly quickly (<100 operations) because
relay C is switching the coil of relay A and this causes arcing at the
normally open terminal of relay C which welds the contacts closed.

I found that this particular design of relay fails after a few 10,000s of operations even when switching non-inductive loads. Generally it fails by remaining closed after power is removed from the coil. I haven't yet worked out what the failure mode is, but some possibilities are:

- PLA creep, reducing the force that pushes the moving pole back to the normally closed contact.

- Magnetization of the coil nail, causing the pole to stick on the normally open contact.